1. Field of the Invention
The present invention relates to a regenerating apparatus for regenerating a digital signal from an input signal, a regenerating method for the regenerating apparatus, and to a receiver including the regenerating apparatus.
2. Description of the Related Art
In the communication field, it is important to increase information transmission capacity. To increase the capacity, a wavelength division multiplexing (WDM) transmission system for transmitting a plurality of signals whose wavelengths are different from one another using a single optical fiber cable is well known. A receiving apparatus using the WDM transmission system includes an identification circuit for comparing an input signal with a predetermined threshold value and converting the input signal to a digital signal based on a result obtained by comparison.
The WDM transmission system is affected by a transmission line loss as in the case with other normal transmission systems. The transmission line loss may be, for example, a loss caused by wavelength dispersion, an SN deterioration, a nonlinear effect, a power fluctuation, or the like. Therefore, deterioration such as distortion, of a waveform of a signal that reaches the receiving apparatus through a transmission line occurs.
When the input signal deteriorates, the identification circuit erroneously identifies the input signal and converts it to an erroneous digital signal. To prevent this, the receiving apparatus includes a forward error correction (hereinafter also referred to as a “FEC”) circuit for correcting an error of the digital signal, which is provided in a subsequent stage of the identification circuit.
Up to now, a threshold value of the identification circuit is constantly fixed. However, when the input signal deteriorates significantly in such a system in which the threshold value is fixed, the erroneous identification of the identification circuit exceeds an error correction capacity of the FEC circuit because of the fixation of the threshold value. Therefore, although the FEC circuit is mounted on the receiving apparatus, a data error caused by the transmission line loss or the like cannot be sufficiently prevented.
In order to solve such a problem, JP 2002-077111 A discloses a regenerating apparatus for changing the threshold value of the identification circuit. In the regenerating apparatus, the threshold value of the identification circuit is not constantly fixed, but is adjusted based on error correction information outputted from the FEC circuit.
Hereinafter, the regenerating apparatus will be described in detail.
FIG. 5 is a block diagram showing a structure of the regenerating apparatus disclosed in JP 2002-077111 A. The regenerating apparatus includes a photoelectric conversion circuit 9 and a threshold value controlling circuit 10.
The photoelectric conversion circuit 9 includes a light receiving element 1, a reshaping circuit 2, a clock reproducing circuit 3, and an identification circuit 4. The light receiving element 1 receives a light signal from a transmission line such as an optical fiber and converts the light signal to an electrical signal. The reshaping circuit 2 amplifies an output signal from the light receiving element 1 up to a predetermined level. The clock reproducing circuit 3 extracts a clock signal from an output signal of the reshaping circuit 2. The identification circuit 4 converts the output signal from the reshaping circuit 2 to a digital signal of “0” or “1” based on a threshold value and performs retiming based on the clock signal outputted from the clock reproducing circuit 3.
The threshold value controlling circuit 10 includes an FEC decoder 5 and an error correction information calculating circuit 6. The FEC decoder 5 performs FEC decoding for error correction on the digital signal outputted from the photoelectric conversion circuit 9. The FEC decoder 5 generates error correction information EB0 indicating the number of bits subjected to error correction from “1” to “0” and error correction information EB1 indicating the number of bits subjected to error correction from “0” to “1”. In this case, the error correction information EB0 and EB1 are changed according to an error correction state, that is, an input signal deterioration state. The error correction information calculating circuit 6 calculates a threshold value based on the error correction information EB0 and EB1 outputted from the FEC decoder 5 and outputs the threshold value to the identification circuit 4 included in the photoelectric conversion circuit 9. The identification circuit 4 converts the output signal from the reshaping circuit 2 to the digital signal of “0” or “1” based on the threshold value.
In other words, the above-mentioned regenerating apparatus changes the threshold value according to the error correction information EB0 and EB1 which are successively varied. Therefore, the occurrence of data error can be suppressed unlike the conventional system in which the threshold value is fixed.
However, the regenerating apparatus continuously changes the threshold value based on the error correction information EB0 and EB1. That is, even when an apparatus abnormality occurs, the regenerating apparatus continues to change the threshold value based on the error correction information EB0 and EB1. In the case of the occurrence of any apparatus abnormality, it is more likely that the respective circuits malfunction. For example, when the FEC decoder 5 or the error correction information calculating circuit 6 malfunction, the threshold value is likely to be calculated as an erroneous value. In some cases, the threshold value becomes an extremely large value (value close to 1) or an extremely small value (value close to 0). Therefore, when the identification circuit 4 converts a received signal to a digital signal based on the erroneously calculated threshold value, an enormous number of data errors may be generated. In other words, when the regenerating apparatus disclosed in JP 2002-077111 A normally operates, it provides sufficient performance. On the other hand, when the apparatus abnormality occurs, it is likely that the performance of the regenerating apparatus becomes lower than that of the conventional system in which the threshold value is fixed. Thus, the regenerating apparatus disclosed in JP 2002-077111 A has a defect that the performance is unstable.